Reduction in size of a transmission unit or a differential unit (hereinafter referred to as “transmission unit or the like”) of an automobile has been demanded for the purpose of reducing fuel consumption and increasing a cabin space. Along with such demands, reduction in torque and reduction in size have been demanded also for a tapered roller bearing which is to be assembled in the transmission unit or the like. In order to achieve the reduction in torque and reduction in size of the tapered roller bearing, increase in load capacity has been demanded. For example, in Patent Literature 1, there is described a tapered roller bearing having a roller coefficient (roller filling ratio) which is set to more than 0.94 to improve the load capacity.
In recent years, there has been an increasing demand for reduction in size of the transmission unit or the like, and further increase in load capacity has been demanded also for a bearing which is to be assembled in the transmission unit or the like. Further, for the purpose of reducing the size of the transmission unit or the like, employment of an aluminum housing and reduction in thickness of a housing have been considered. In this case, the stiffness of the entire unit is reduced, and a large moment load is applied to the tapered roller bearing. Consequently, a load condition of the tapered roller bearing becomes stricter. Further, when the aluminum housing is employed, the amount of thermal expansion of the housing is increased, with the result that reduction in preload (which is so-called “preload loss”) becomes more liable to occur in the tapered roller bearing. Therefore, more highly functional tapered roller bearing is demanded.
As described above, the demands on the tapered roller bearing have become stricter, and there is difficulty in meeting the demands by only increasing the roller coefficient as in Patent Literature 1.
As a method for further increasing the load capacity of the tapered roller bearing, for example, there has been known a method involving forming each of raceway surfaces of an inner ring and an outer ring into a crowning shape. For example, in Patent Literature 2, there is described a raceway surface including a so-called composite crowning surface. In the composite crowning surface, an arcuate curve having a large curvature radius (large-arc portion) is formed at a center portion of the raceway surface in a generating-line direction, and arcuate curves each having a small curvature radius (small-arc portions) are formed at both end portions of the raceway surface in the generating-line direction. As described above, with the raceway surface including the composite crowning surface, when the large-arc portion formed at the center of the raceway surface and the roller are brought into contact with each other during the normal use, a contact length between the raceway surface and the roller is increased to reduce a surface pressure, thereby being capable of preventing defects such as surface-originating separation in an early stage. Meanwhile, with the small-arc portions formed at the end portions of the raceway surface, the end portions can be recessed toward the side apart from the roller. Therefore, for example, even when a high load is applied to the tapered roller bearing, the contact between the raceway surface of the outer ring and the end portions of the tapered roller is avoided as much as possible, thereby being capable of preventing generation of an excessive edge load.